Fluorine is an essential element for producing fluorochemicals and fluoropolymers and is currently derived, primarily, from fluorspar, a mineral which is a crystalline form of calcium fluoride. However, world-wide reserves of fluorspar are being depleted and new economical sources of fluorine are desired.
An important reserve of fluorine is phosphate rock which contains a variety of apatites including fluorapatite, i.e., CaF.sub.2.3Ca.sub.3 (PO.sub.4).sub.2, a mineral which is used for the manufacture of phosphoric acid. This mineral constitutes a fluorine reserve which is about four times greater than the proven reserves of fluorspar. During the manufacture of phosphoric acid most of the fluorine is removed as fluosilicic acid (FSA). There is some demand for FSA for fluoridating drinking water and for the manufacture of cryolite and aluminum fluoride. However, because this demand is small, most of the FSA produced during phosphoric acid manufacture is wasted, e.g., it is sent to a waste-water pond with the fluorine ending up in the surrounding environment and can cause an environmental pollution problem.
Over the years numerous processes, several of which are described below, have been developed to recover the fluorine from phosphate minerals. The United States Bureau of Mines (Chem. Abst., 75:23270, 1971) has shown how waste fluosilicic acid can be converted to an acid-grade fluorspar (CaF.sub.2). A first step involves treating the FSA with ammonia to precipitate silica, which is removed by filtration to form NH.sub.4 F. In a second step ammonium fluoride is treated with lime to form CaF.sub.2.
U.S. Pat. No. 5,531,975 describes a process for reacting phosphate rock and FSA to produce a slurry comprising phosphoric acid, calcium fluoride, silicon dioxide and undigested phosphate rock. An excess stoichiometric amount of calcium to fluoride is initially present in the slurry. The product slurry is pumped into a vacuum filter or centrifuge where the phosphoric acid and colloidal calcium fluoride are separated from the undigested phosphate rock and silic. In Example 1 of the '975 patent, the weight ratio of F:Si in the product (initial filtrate) is shown to be about 30:1.
Surprisingly, a process has been discovered which yields a much more efficient process for producing calcium fluoride from FSA, with much higher recovery of fluorine from the FSA and calcium fluoride containing much lower levels of silica than previously.